The frog neuromuscular junction will be used to study synaptic remodelling and the elimination of polyneuronal innervation during development. A new technique of vital staining with fluorescent dyes will be used to directly observe identified motor nerve terminals in living, anesthetized animals. Observations will be verified by conventional histology and electrophysiological recording. The first aim is to study motor nerve terminal remodelling in living adult frogs. There is substantial evidence that such remodelling occurs, but this evidence is from histological studies of fixed tissue. Through direct observation, this dynamic process can be described more thoroughly and the validity of commonly used histological criteria tested. The second aim is to observe nerve terminal remodelling as it is associated with the robust muscle growth that occurs in frogs during early adult life. Terminal remodelling takes two forms. First, terminals lengthen as target muscle fibers increase in diameter. Second, discrete new synaptic sites are added as fibers increase in length. The role of this remodelling in the maintenance of synaptic efficacy and in nerve-target trophic interactions will be explored. The third aim is to study developmental synapse elimination, using a unique adult preparation that offers several experimental advantages. Results will be used to test the hypothesis that competitive interactions between multiple nerve terminals at the same postsynaptic site are influenced by spatial proximity. The development and application of the innovative technique of in situ observation will significantly advance our understanding of synaptic plasticity at this accessible peripheral synapse. Based on historical precedent, results should be directly applicable to similar phenomena occurring in the brain and spinal cord. Such plasticity is likely to be of major importance in determining the ability of the nervous system to recover from injury or disease, and may be fundamentally involved in mechanisms of memory and learning.